1. Field of the Invention
The present invention relates to a belt-drive system for driving auxiliary devices mounted on an automotive vehicle.
2. Description of Related Art
In a recent automotive vehicle, a space available for an engine compartment is getting smaller in order to secure a sufficient space for a passenger compartment. To minimize a space occupied by a belt-drive system for driving various auxiliary devices including a generator, a so-called serpentine belt-drive system that drives all the auxiliary devices with a single belt is becoming popular. An example of such a serpentine belt-drive system is shown in FIG. 7. A single driving belt 31 is wound around a crankshaft pulley 40 connected to an internal combustion engine and all other pulleys to be driven by the engine. A pulley 20a is for driving a generator 2a, a pulley 50 for a water pump, and a pulley 60 for a compressor of an air-conditioner. A pulley 30 of an automatic belt-tensioner 3 controls a tension of the driving belt 31. All the pulleys connected to respective auxiliary devices are driven by the engine through a single driving belt.
A generator is required to output a higher power to cover increasing electric loads for comfort and safety of passengers. Accordingly, it is unavoidable to increase a size of the generator and its rotor. As the rotor size increases, an inertia moment of the rotor also increases. In some cases, it is required to supply a high power to heat-generating loads such as a heated windshield or a sheet-heater. It has been proposed to use a 42-volt system instead of a conventional 14-volt system for supplying high power to such heat-generating loads.
On the other hand, there is a tendency to set an idling speed of an engine to a lower level to cope with exhaust gas problems. Further, in case of a diesel engine, a common rail system, in which a pressure in a combustion chamber is considerably increased for decreasing pollutants in exhaust gas, is getting popular in the market.
A driving torque of the crankshaft pulley fluctuates in a pulsating manner according to strokes of the engine, i.e., in synchronism with an explosion frequency of the engine. Therefore, an instantaneous rotational speed of the crankshaft fluctuates. Especially, during an engine idling period in which the driving torque of the engine is small and unstable, the engine speed fluctuation is larger than that of other situations. When the idling speed is further lowered to cope with the exhaust gas problems, the rotational speed of the engine becomes further unstable, and the fluctuations in the rotational speed of the crankshaft pulley increase.
The auxiliary devices driven by the crankshaft pulley have respective inertia moments. Therefore, the rotational speed of each auxiliary device cannot instantaneously follow the fluctuations in the rotational speed of the crankshaft pulley. The tension of the driving belt becomes high when the driving belt is pulled by the crankshaft pulley according to its instantaneously increasing rotational speed. On the other hand, the tension of the driving belt becomes low when the rotational speed of the crankshaft pulley instantaneously decreases. The phenomenon mentioned above occurs at an upstream side of the driving belt with respect to the crankshaft pulley. An opposite phenomenon occurs at an downstream side of the driving belt. In short, the driving belt becomes repeatedly loose and tight according to the fluctuations in the rotational speed of the crankshaft pulley. The larger the rotational speed fluctuation becomes, the larger the belt tension fluctuation becomes.
The belt tension fluctuation is highly affected by the generator than by other auxiliary devices, because the rotor of the generator has a large inertia moment and a high pulley ratio. Therefore, a large fluctuation in the inertia toque (the inertia torque is a product of the inertia moment and acceleration or deceleration in the rotational speed) is involved in the generator. As the belt tension fluctuation becomes large, the belt is repeatedly loosened and tightened in a higher amount. This causes slippage between the belt and the pulleys, and a durability life of the belt is shortened.
In addition, when the fluctuation in the belt tension becomes large, the automatic belt-tensioner used in a serpentine belt-drive system widely swings to maintain the belt tension constant. The belt-tensioner may interfere with other devices, thereby generating noises and causing damages in the belt-tensioner and/or other devices. Especially, in the case where the belt-drive system is driven by a diesel engine, the fluctuations in its rotational speed tend to become larger because the pressure in the combustion chamber becomes higher as mentioned above. Therefore, the above problems are further serious for the diesel engine.
To cope with these problems, JP-B2-7-72585 proposes to use a one-way clutch in a pulley of a generator. Driving torque transmission between the crankshaft pulley and the generator is interrupted by the one-way clutch when the rotational speed of the crankshaft pulley is decreasing. When the rotational speed of the crankshaft pulley is increasing, the generator pulley is not coupled to the rotor of the generator until the generator pulley speed becomes equal to that of the rotor. Thus, the inertia torque of the rotor is prevented from being transmitted to the generator pulley by the one-way clutch, and the fluctuations in the belt tension are alleviated.
In the one-way clutch shown in JP-B2-7-72585, the torque transmission or interruption between the rotor and the pulley is carried out by rollers or sprags disposed in the one-way clutch. A considerable amount of mechanical stress is imposed on the rollers or the sprags in operation of the one-way clutch. On the other hand, it is required for the generator to generate a higher power at an idling speed which is lowered to improve exhaust gas. For this purpose, it is necessary to reduce the diameter of the generator pulley and to increase the pulley ratio. It is contradictory to a down-sizing requirement to make the axial length of the generator pulley longer. Therefore, the one-way clutch has to be made small in size. To make the one-way clutch small in size, it is necessary to make its components small and to reduce an amount of lubricant contained in the one-way clutch. This results in sacrificing the durability of the one-way clutch. Further, it is difficult to enlarge the size of the generator because the engine compartment space is limited.
JP-A-2001-309574 proposes to provide an additional generator for supplying power to high voltage loads in addition to a generator for supplying power to conventional voltage loads. It also proposes to provide a circuit for exchanging power between two systems operating under different voltages. Though JP-A-2001-309574 shows an idea to use two generators, nothing about using those generators for reducing the fluctuations in the belt-tension is mentioned or shown.